The present invention relates to a capacitor post for use within a large high voltage capacitor, and more particularly to a capacitor core post for use within a capacitor that has electrically insulative properties while also having thermally conductive properties to improve the heat dissipating properties between the internal active elements of the capacitor foil roll and the package (or can) in which the capacitor is encased. The improved capacitor construction of the present invention provides improved heat dissipation for a capacitor and an economical construction adaptable to automated large-scale manufacturing.
Electrolytic capacitors, and specifically aluminum electrolytic capacitors, generate internal heat during operation because of fluctuating current (xe2x80x9cripple currentxe2x80x9d) and internal resistance (Effective Series Resistancexe2x80x94xe2x80x9cESRxe2x80x9d). The heat is generated internally in the active element of the roll and must diffuse outward to the packaging (or can) before it can be carried away by convection, conduction and/or radiation to the ambient environment. Radial and axial heat flows serve to conduct the heat from the core of the capacitor to the sides and bottom of a cylindrical package in which the capacitor may be encased. Construction details of the capacitor can facilitate or introduce resistance to these heat flows. Thus, it is desirable to use construction designs for capacitors which minimize the addition of internal resistance and which facilitate the conduction of heat from the core to the outside environment.
Traditionally, the capacitance of a capacitor, among other parameters, determines the required dimensions and size of the roll. In the past, the smallest arbor hole (or center opening) possible was used to maximize the amount of foil in the capacitor roll while minimizing the size of the package or can required to enclose the roll, thus producing a dense roll that allowed little heat to dissipate through the arbor hole. In addition, a common construction technique employed with this design involved the use of a xe2x80x9cpottingxe2x80x9d material between the capacitor and the can to aid in anchoring the roll. The practice involves embedding the roll in asphalt, wax, etc. While potting anchors the roll, it does not promote efficient heat flow from the roll to the can, because the potting materials are generally insulators. Accordingly, the heat must pass through the large thermal resistance of the insulating potting compound before being diffused to the package.
Another known construction technique involves the use of indentations in the can or packaging of the capacitor to assist in anchoring the roll and to promote heat transfer through the increased contact surface area between the can side wall and the roll at multiple points (called xe2x80x9crilled constructionxe2x80x9d). However, a paper layer is normally wound around the outer surface of the active element (the portion of the roll where the two electrodes are present) and is thus interposed between the active layers of the roll and the can. This interposed paper layer contributes to the internal thermal resistance. This practice is particularly detrimental when the active element is small and several layers of paper must be used.
Other methods of construction employ a variable arbor hole to control the roll diameter for a snug fit into the can and leave a predetermined void volume in the center of the roll for gas buildup. The use of a large variable arbor has proven to be impractical and abandoned as a commercial technique due to difficulties in process control and collapse of the resulting un-supported arbor hole (which leads to elevated ESR of the capacitor). In addition, the close control of the roll diameter required a varying number of extra turns of paper on the exterior of the roll (contributing to heat buildup) unless a very highly variable mandrel was used. Moreover, an adequate press fit between the roll and the can is almost impossible to manufacture when a variable mandrel was used. Without an extremely good press fit, small air gaps result between the can and the surface of the roll and act as insulators, thus contributing to internal resistance.
Another technique describes the use of a small air gap between the exterior of the roll and the can with indenting (rilling) of the can to force mechanical and thermal contact at a number of points around the periphery of the roll. In addition, an indentation in the can bottom was included to help support the arbor hole against collapse. A low profile roll was also used in an attempt to shorten the axial thermal path to the can bottom and to limit the collapse of the arbor hole. Again, this construction suffers from several commercial disadvantages and was never widely practiced due, in part, to manufacturing implementation problems and arbor hole collapse.
Presently, a number of manufacturers offer variations of the techniques set forth above. The variations focus on extracting heat from the roll via extended cathode foils that reach and contact the can bottom. These designs provide for rolls that mostly fill the can and are wound on standard diameter mandrels; therefore, there are air gaps around the roll periphery. The cathodes are extended down beyond the anode foil to contact the can bottom. The roll is anchored by compression between the cover and the can bottom. Currently available capacitors based on the extended cathode and the rilled can construction techniques typically do not incorporate large arbor holes. Instead, extra paper is wound around the outside of the active element to permit the can to be indented (rilled) for anchoring the roll. The disadvantage of these practices is that the heat dissipation is mainly forced into the axial direction down the cathode foils to the can bottom. Thus, these designs suffer by stifling radial heat flow.
In an effort to employ the above techniques, yet minimize the occurrence of collapsing the central arbor hole, prior art capacitors have employed metallic posts inserted within the center of the capacitor. The post thereby provides a solid core to prevent collapse of the arbor hole while providing a thermal path through which heat can be dissipated. The capacitor posts of the prior art are typically made of a metallic material and typically include a plate at the lowermost end. However, these prior art capacitor posts and mounting plates must be electrically insulative for proper operation of the capacitor. The metallic prior art capacitor post/plate structure is commonly powder coated to make it electrically insulative. However, such powder coating is expensive and time consuming to carry out.
Therefore, there is a need for an electrolytic capacitor support core and a method of constructing the same in which internal thermal resistances are reduced and heat dissipation from the core or center of a capacitor to the packaging is increased. There is also a need for a capacitor core that is injection moldable from thermally conductive, electrically insulative polymer materials to provide a more effective path for transfer of heat from the active element of the capacitor.
The present invention provides for a novel assembly and method of constructing an electrolytic capacitor in which the dissipation of heat from the active elements of the capacitor is increased. The capacitor post assembly of the present invention has many advantages over prior art posts in that it maintains the metallic core for support of the arbor hole in the roll and for heat transfer from the core of the capacitor in addition to having an integrally formed electrical insulation component. The post is preferably insert injection molded from thermally conductive polymer materials, which enables the part to be inexpensively made in large quantities. However, other molding techniques known in the art may also be used.
The method of the present invention provides a method for manufacturing a capacitor core that includes fabricating an inactive core element to one or more pre-determined dimensions and net shape insert molding a layer of thermally conductive, electrically insulative polymer over the core element. In addition, a thermally conductive transfer pad is integrally molded with the polymer layer to improve thermal transfer from the capacitor when assembled and installed in an electronic assembly. The composite core element may be an arbor upon which the capacitor element is wound, capable of remaining in the capacitor after assembly without affecting operation of the capacitor. In addition, the core element may be inserted into an opening within the outer can housing of the capacitor and fastened therein.
The invention also provides an improved core element for an electrolytic capacitor consisting of an inactive core element; and a net shape injection molded coating thereon having thermally conductive and electrically insulative properties and an integrally molded thermal transfer interface pad. The molding material of the present invention is a dielectric thermoplastic, such as an elastomeric polymer base, with a thermally conductive filler loaded therein. The filler may be carbon fiber, copper flakes, boron nitride, or the like. Since the polymer base is a dielectric, the molded portion of the post will be electrically insulative while still being thermally conductive.
The outer layer of the capacitor post of the present invention is preferably injected molded from a net shape moldable thermally conductive polymer composition. As the protective layer of the present invention, which is in contact with the other elements of the capacitor, is fabricated from elastomeric polymer materials it is lighter than the all metallic posts of the prior art and does not require machining to reach its final dimension. The present invention also has the advantage of being net shape moldable, meaning, the part that is created in the injection molding process does not require any further processing steps after it is removed from the mold and before it is incorporated into the finished device. Both the lighter weight and the net-shape moldability are distinct advantages over the prior art where metallic parts had to be machined in several steps to achieve the desired part geometry.
Therefore, it is an object of the present invention to provide a capacitor post structure that can be employed without powder coating. It is also an object for such a structure to be thermally conductive to assist in the cooling of the capacitor during operation both through providing a thermal transfer pathway to the central core and an integral thermal transfer interface pad between the capacitor can and the mounting substrate onto which the capacitor is mounted. It is also an object of the present invention to provide an electrolytic capacitor, which increases the efficiency of the flow of heat from the active elements of a capacitor in both the radial and axial directions. It is also an object of the invention to provide a support for the arbor hole, which is highly thermally conductive while also being electrically insulative. Further, it is an object of the invention to provide a capacitor support post construction, which can be easily and inexpensively manufactured.